1. Field of the Invention
The present invention relates to a novel lithium secondary battery electrolyte capable of providing a lithium secondary battery superior in the battery cycle characteristics and in battery characteristics such as the electric capacity and storage characteristics and to a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have come into wide use as sources for driving compact electronic devices. A lithium secondary battery is mainly composed of a cathode, a non-aqueous electrolyte, and an anode. In particular, a secondary battery having a lithium complex oxide such as LiCoO2 as its cathode and a carbonaceous material or lithium metal as its anode is preferably used. As the electrolyte for this lithium secondary battery, carbonates such as ethylene carbonate (EC), propylene carbonate (PC) are preferably used.
However, a secondary battery having more superior battery cycle characteristics and battery characteristics such as the electric capacity, is being sought.
In a lithium secondary battery using, for example, LiCoO2, LiMn2O4, LiNiO2, or the like as the cathode active material, the solvent in the non-aqueous electrolyte is locally partially oxidized and decomposed at the time of charging. The decomposed product inhibits the desirable electrochemical reaction of the battery, and therefore, causes the reduction in the battery performance. This is believed to be due to the electrochemical oxidation of the solvent at the interface of the cathode material and non-aqueous electrolyte.
Further, in a lithium secondary battery using, for example, highly crystallized carbonaceous material such as natural graphite, artificial graphite, as the anode active material, exfoliation of the carbonaceous material is observed. Depending on the extent of the phenomenon, the capacity sometimes becomes irreversible. This exfoliation occurs due to the decomposition of the solvent in the electrolyte at the time of charging and is due to the electrochemical reduction of the solvent at the interface of the carbonaceous anode material and the electrolyte.
Further, in general, a carbonaceous anode has a capacity called an xe2x80x9cirreversible capacityxe2x80x9d in which part of the lithium inserted in the carbonaceous anode at the time of the first charging will not disassociate at the time of discharging. To prepare a battery of a predetermined capacity, it is necessary to use an excess amount of the cathode active material corresponding to the amount of the irreversible capacity, and therefore, there are the problems that the battery weight becomes heavier by that amount or extra cost is required.
As mentioned above, the battery cycle characteristics and battery characteristics such as the electric capacity are not necessarily satisfactory at the present time.
The objects of the present invention are to solve the above-mentioned problems relating to lithium secondary battery electrolytes in the prior art and to provide a lithium secondary battery electrolyte capable of constructing a lithium secondary battery superior in battery cycle characteristics and further superior in battery characteristics such as the electric capacity and storage characteristics in a charged state and a lithium secondary battery using the same.
In accordance with the present invention, there is provided a lithium secondary battery electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one compound having the formula (I): 
wherein, R1 and R2 independently indicate, a C1 to C12 alkyl group, a C3 to C6. cycloalkyl group, or an aryl group, x indicates an oxygen atom or a sulfur atom, and n indicates an integer of 1 or 2.
In accordance with the present invention, there is also provided a lithium secondary battery comprising a cathode, an anode, and an electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one compound having the formula (I): 
wherein, R1 and R2 independently indicate, a C1 to C12 alkyl group, a C3 to C6 cycloalkyl group, or an aryl group, wherein X indicates an oxygen atom or a sulfur atom, and n indicates an integer of 1 or 2.
In accordance with the present invention, there is further provided a lithium secondary battery electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, said electrolyte containing at least one thiol salt having the formula (II):
R3xe2x80x94Sxe2x80x94Mxe2x80x83xe2x80x83(II)
wherein, R3 indicates a C1 to C15 alkyl group or a C3 to C12 cycloalkyl group which may be substituted with at least one C1 to C4 alkyl group, a C7 to C15 benzyl group which may be substituted with at least one C1 to C4 alkyl group, or a C6 to C15 aryl group, which may be substituted with at least one C1 to C4 alkyl group, M indicates an alkali metal, and R3 may be substituted with at least one halogen atom.
In accordance with the present invention, there is further provided a lithium secondary battery comprising a cathode, an anode, and a lithium salt comprising a non-aqueous solvent and an electrolyte dissolved therein, said electrolyte containing at least one thiol salt having the formula (II):
R3xe2x80x94Sxe2x80x94Mxe2x80x83xe2x80x83(II)
wherein, R3 indicates a C1 to C15 alkyl group or a C3 to C12 cycloalkyl group which may be substituted with at least one C1 to C4 alkyl group, a C7 to C15 benzyl group which may be substituted with at least one C1 to C4 alkyl group, or a C6 to C15 aryl group, which may be substituted with at least one C1 to C4 alkyl group, M indicates an alkali metal, and R3 may be substituted with at least one halogen atom.
The compound having the general formula (I) contained in the electrolyte is reduced and decomposed before the organic solvent in the electrolyte at the carbon anode surface at the time of charging. Part of the decomposed product forms a passivation film at the carbon anode surface made highly crystalline by activity such as natural graphite or artificial graphite to thereby, it is believed, prevent in advance the reduction and decomposition of the organic solvent in the electrolyte.
Further, part of the decomposed product is oxidized and decomposed before the organic solvent in the electrolyte at the slight overvoltage portion where the potential of the surface of the cathode material becomes excessively high to thereby, it is believed, prevent in advance the oxidation and decomposition of the organic solvent in the electrolyte.
Due to the above phenomina, it is believed that there is the effect of suppressing the decomposition of the electrolyte, without impairing from the normal reaction of the battery.
In the compound contained in the electrolyte composed of a non-aqueous solvent and a lithium salt dissolved therein, the R1 and R2 in the compound having the general formula (I) preferably independently represent a C1 to C12 alkyl group such as a methyl group, ethyl group, propyl group, butyl group, pentyl group, and hexyl group. The alkyl group is preferably a branched alkyl group such as an isopropyl group and isobutyl group. Further, the cyclopropyl group may be a C3 to C6 cycloalkyl group such as a cyclohexyl group. Further, it may contain a C6 to C12 aryl group such as a phenyl group, benzyl group, or p-tolyl group. Further, X represents an oxygen atom or sulfur atom, wherein n indicates an integer of 1 or 2.
As specific examples of the compound having the general formula (I), for example, when X is an oxygen atom, S-methyl O-methyl thiocarbonate (R1=methyl group, R2=methyl group, n=1), S-ethyl O-methyl thiocarbonate (R1=ethyl group, R2=methyl group, n=1), S-butyl O-methyl thiocarbonate (R1=butyl group, R2=methyl group, n=1), S-cyclohexyl O-methyl thiocarbonate (R1=cyclohexyl group, R2=methyl group, n=1), S-phenyl O-methyl thiocarbonate (R1=phenyl group, R2=methyl group, n=1), S-phenyl O-ethyl thiocarbonate (R1=phenyl group, R2=ethyl group, n=1), S-phenyl O-cyclohexyl thiocarbonate (R1=phenyl group, R2=cyclohexyl group, n=1), S-phenyl O-phenyl thiocarbonate (R1=R2=phenyl group, n=1), S-p-tolyl O-methyl thiocarbonate (R1=p-tolyl group, R2=methyl group, n=1), S-methyl O-methyl thiooxalate (R1=methyl group, R2=methyl group, n=2), S-ethyl O-methyl thiooxalate (R1=ethyl group, R2=methyl group, n=2), S-butyl O-methyl thiooxalate (R1=butyl group, R2=methyl group, n=2), S-cyclohexyl O-methyl thiooxalate (R1=cyclohexyl group, R2=methyl group, n=2), S-phenyl O-methyl thiooxalate (R1=phenyl group, R2=methyl group, n=2), S-phenyl O-ethyl thiooxalate (R1=phenyl group, R2=ethyl group, n=2), S-phenyl O-cyclohexyl thiooxalate (R1=phenyl group, R2-cyclohexyl group, n=2), S-phenyl O-phenyl thiooxalate (R1=R2=phenyl group, n=2), and S-p-tolyl O-methyl thiooxalate (R1=p-tolyl group, R2=methyl group, n=2) may be mentioned.
For example, when X=sulfur atom, S,S-dimethyl dithiocarbonate (R1=methyl group, R2=methyl group, n=1), S-ethyl S-methyl dithiocarbonate (R1=ethyl group, R2=methyl group, n=1), S-butyl S-methyl dithiocarbonate (R1=butyl group, R2=methyl group, n=1), S-cyclohexyl S-methyl dithiocarbonate (R1=cyclohexyl group, R2=methyl group, n=1), S-phenyl S-methyl dithiocarbonate (R1=phenyl group, R2=methyl group, n=1), S-phenyl S-ethyl dithiocarbonate (R1=phenyl group, R2=ethyl group, n=1), S-phenyl S-cyclohexyl dithiocarbonate (R1=phenyl group, R2=cyclohexyl group, n=1), S-phenyl S-phenyl dithiocarbonate (R1=R2=phenyl group, n=1), S-p-tolyl S-methyl dithiocarbonate (R1=p-tolyl group, R2=methyl group, n=1), S-methyl O-methyl dithiooxalate (R1=methyl group, R2=methyl group, n=2), S-ethyl S-methyl dithiooxalate (R1=ethyl group, R2=methyl group, n=2), S-butyl S-methyl dithiooxalate (R1=butyl group, R2=methyl group, n=2), S-cyclohexyl S-methyl dithiooxalate (R1=cyclohexyl group, R2=methyl group, n=2), S-phenyl S-methyl dithiooxalate (R1=phenyl group, R2=methyl group, n=2), S-phenyl S-ethyl dithiooxalate (R1=phenyl group, R2=ethyl group, n=2), S-phenyl S-cyclohexyl dithiooxalate (R1=phenyl group, R2=cyclohexyl group, n=2), S-phenyl S-phenyl dithiooxalate (R1=R2=phenyl group, n=2), S-p-tolyl S-methyl dithiooxalate (R1=p-tolyl group, R2=methyl group, n=2) may be mentioned. The present invention, however, is not limited to these compounds in any way.
In the compound, if the content of the compound having the general formula (I) is too large, the electroconductivity etc. of the electrolyte changes and the battery performance falls in some cases. Further, if too small, a sufficient film will not be formed and the anticipated battery properties cannot be obtained. Therefore, the content is preferably in the range of 0.01 to 10% by weight, more preferably 0.02 to 8% by weight, particularly 0.1 to 5% by weight, based upon the weight of the electrolyte.
The compound (II) contained in the electrolyte is believed to be oxidized at the cathode surface at the time of charging to form a passivative film at the cathode surface and suppress the oxidation and decomposition of the electrolyte on the cathode and thereby improve the cycle characteristics and storage characteristics of the battery. Further, electrons are discharged when the compound (II) contained in the electrolyte is oxidized on the cathode, and therefore, those electrons can be used for storage of M+ at the anode side and thus a battery with a higher capacity than one in the case of an electrolyte not containing the compound (II) can be fabricated.
In the compound contained in the electrolyte composed of a non-aqueous solvent and a lithium salt contained therein, the R3 in the compound having the general formula (II) represents a C1 to C15 alkyl group or a C3 to C12 cycloalkyl group which may be substituted with at least one C1 to C4 alkyl group, a C7 to C15 benzyl group which may be substituted with at least one C1 to C4 alkyl group, or C6 to C15 aryl group which may be substituted with at least one C1 to C4 alkyl group, and M represents an alkali metal. Further, R3 may be substituted with at least one halogen atom selected from the group consisting of fluorine, chlorine, bromine, and iodine.
As the C1 to C15 alkyl group, a C1 to C15 linear or branched alkyl group such as a methyl group, ethyl group, propyl group, iso-propyl group, n-butyl group, iso-butyl group, or tert-butyl group or alkyl group substituted with a halogen atom such as a 3-chloropropyl group, 3-bromopropyl group, 3-fluoropropyl group, or 4-chloro-n-butyl group may be mentioned.
Further, a C3 to C12 cycloalkyl group such as a cyclopropyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group, cyclooctyl group, or cyclododecyl group or a C3 to C12 cycloalkyl group with at least one hydrogen atom on the cycloalkyl ring substituted with a C1 to C4 alkyl group or halogen atom such as a 4-methylcyclohexyl group, 4-chlorocyclohexyl group, 4-bromocyclohexyl group, or 4-fluorocyclohexyl group may be mentioned.
Further, a benzyl group or a C7 to C15 benzyl group with at least one hydrogen atom on the benzene ring substituted with a C1 to C4 alkyl group or halogen atom such as a 4-methylbenzyl group, 4-chlorobenzyl group, 4-bromobenzyl group, or 4-fluorobenzyl group may be mentioned.
Further, an aryl group such as a phenyl group or naphthyl group or a C6 to C15 aryl group with at least one hydrogen atom on the benzene ring or naphthalene ring substituted with a C1 to C4 alkyl group or halogen group such as a p-tosyl group, 4-chlorophenyl group, 4-bromophenyl group, 4-fluorophenyl group, 2-methylnaphthyl group, 2-chloronaphthyl group, 2-bromonaphthyl group, or 2-fluoronaphthyl group may be mentioned.
M may be Na, K, Li, etc. In particular, Li is preferable since it is the basest metal element electrochemically.
As specific examples of the compound having the formula (II), lithium methanethiolate, lithium ethanethiolate, lithium n-propanethiolate, lithium 1-methylethanethiolate, lithium n-butanethiolate, lithium 2-methylpropanethiolate, lithium 1,1-dimethylethanethiolate, lithium 2,2-dimethylpropanethiolate, lithium pentanethiolate, lithium hexanethiolate, lithium heptanethiolate, lithium octanethiolate, lithium decanethiolate, lithium dodecanethiolate, lithium 3-chloro-n-propanethiolate, lithium 3-bromo-n-propanethiolate, lithium 3-fluoro-n-propanethiolate, lithium 4-chloro-n-butanethiolate, sodium butanethiolate, potassium butanethiolate, lithium cyclopentanethiolate, lithium cyclohexanethiolate, lithium cycloheptanethiolate, lithium cyclooctanethiolate, lithium cyclodecanethiolate, lithium cyclododecanethiolate, lithium 4-methylcyclohexanethiolate, lithium 4-chlorocyclohexanethiolate, lithium 4-bromocyclohexanethiolate, lithium 4-fluorocyclohexanethiolate, sodium cyclohexanethiolate, potassium cyclohexanethiolate, lithium phenylmethanethiolate (C6H5CH2-S-Li), lithium 4-methylphenylmethanethiolate (4-CH3-C6H4CH2-S-Li ), lithium 4-chlorophenylmethanethiolate, lithium 4-bromophenylmethanethiolate, lithium 4-fluorophenylmethanethiolate, sodium phenylmethanethiolate, potassium phenylmethanethiolate, lithium benzenethiolate, lithium 2-methylbenzenethiolate, lithium 3-methylbenzenethiolate, lithium 4-methylbenzenethiolate, lithium 2,4-dimethylbenzenethiolate, lithium 2,5-dimethylbenzenethiolate, lithium 2,6-dimethylbenzenethiolate, lithium 3,4-dimethylbenzenethiolate, lithium 3,5-dimethylbenzenethiolate, lithium 2-ethylbenzenethiolate, lithium 2-iso-propylbenzenethiolate, sodium benzenethiolate, sodium 2-methylbenzenethiolate, sodium 3-methylbenzenethiolate, sodium 4-methylbenzenethiolate, potassium benzenethiolate, potassium 2-methylbenzenethiolate, potassium 3-methylbenzenethiolate, potassium 4-methylbenzenethiolate, lithium 4-chlorobenzenethiolate, lithium 4-bromobenzenethiolate, lithium 4-fluorobenzenethiolate, lithium naphthalenethiolate, lithium 2-methyl-1-naphthalenethiolate, lithium 2-chloro-1-naphthalenethiolate, lithium 2-bromo-1-naphthalenethiolate, lithium 2-fluoro-1-naphthalenethiolate, sodium naphthalenethiolate, potassium naphthalenethiolate, etc. may be mentioned.
The present invention, however, is not limited to these compounds in any way.
In the compound, if the content of the compound of formula (II) is too large, the passivative film formed on the cathode surface becomes too thick, movement of Li ions is inhibited, the battery performance falls or the electroconductivity etc. of the electrolyte changes and the battery performance falls in some cases. Further, if too small, a sufficient film is not formed and the anticipated battery properties cannot be obtained. Therefore, the content is preferably in the range of 0.01 to 10% by weight, particularly 0.05 to 5% by weight, based upon the weight of the electrolyte.
The non-aqueous solvents usable in the present invention preferably include high dielectric constant solvents and low viscosity solvents.
As a high dielectric constant solvent, for example, a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC) may be suitably mentioned. These high dielectric constant solvents may be used alone or any combinations thereof.
As the low viscosity solvent, for example, a linear carbonate such as dimethyl carbonate (DMC), methylethyl carbonate (MEC), or diethyl carbonate (DEC), an ether such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, or 1,2-dibutoxyethane, a lactone such as xcex3-butyrolactone, a nitrile such as acetonitrile, an ester such as methyl propionate, and an amide such as dimethylformamide may be mentioned. These low viscosity solvents may be used alone or any combinations thereof.
Any of the high dielectric constant solvents and the low viscosity solvents are selected and combined for use. Note that the high dielectric constant solvent and the low viscosity solvent are used in a volume ratio (high dielectric constant solvent:low viscosity solvent) of preferably 1:9 to 4:1, more preferably 1:4 to 7:3.
As the lithium salt usable in the present invention, for example, LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(So2C2F5)2, LiC(SO2CF3)3, etc. may be mentioned. These lithium salts may be used alone or any combinations thereof. These lithium salts are used added to the non-aqueous solvent in a concentration of preferably 0.1 to 3M, more preferably 0.5 to 1.5M.
The electrolyte of the present invention is obtained by, for example, mixing the above high dielectric constant solvents or low viscosity solvents, adding the lithium salt to the resultant solution, then dissolving at least one compound having the general formula (I) or (II) therein.
The electrolyte of the present invention is suitably used as a component of a secondary battery, in particular the component of a lithium secondary battery. The components other than the electrolyte constituting the secondary battery are not particularly limited. Various components used in the prior art may be used.
For example, as the cathode active material, a complex metal oxide of at least one type of metal selected from the group comprising cobalt, manganese, nickel, chromium, iron, and vanadium with lithium is used. As such a complex metal oxide, for example, LiCoO2, LiMn2O4, LiNiO2, etc. may be mentioned.
The cathode is prepared by mixing the cathode active material with a conductive agent such as acetylene black or carbon black and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) and 1-methyl-2-pyrrolidine to make a cathode paste, then coating (or pressing) this cathode material on aluminum foil or stainless steel foil or lath serving as a collector, followed by drying, pressing, and heating the same at a temperature of about 50 to 250xc2x0 C. for, for example, about 2 hours under a vacuum.
As the anode active material, a lithium metal or lithium alloy and a carbonaceous material having a graphite-type crystal structure capable of intercalate-disintercalate lithium (pyrolyric carbons, coke, graphite (or artificial graphite, natural graphite, etc.), organic polymer compound sintered fines, carbon fiber), complex tin oxides, and other substances are used. In particular, a carbonaceous material having a graphite-type crystal structure having a lattice spacing (d002) of a lattice plane (002) of 0.335 to 0.340 nm (nanometers) is preferably used. Note that a powder material such as a carbonaceous material is kneaded with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF) for use as an anode paste.
The configuration of the lithium secondary battery is not particularly limited. A coin battery having a cathode, anode, and single or double layer separator or a cylindrical battery, prismatic battery, etc. having a cathode, anode, and roll-like separator may be mentioned as examples. Note that as the separator, a known polyolefin microporous membrane, woven fabric, nonwoven fabric, etc. may be used.